Blue Stagglers
In Astronomy, a blue staggler is a type of star that is more luminous then surrounding stars from a Globular Cluster or another region with crowded stars. The most accepted theory is that blue stagglers are the result of merging stars. By extrapolating, the term blue staggler can also be used to any star creating from the merger of two or more stars. The theory is that in crowded stellar neighbourhoods stars are formed at the same time. As brighter stars exhaust their fuel fast, they die first. Therefore, the age of a globular cluster can be estimated based on the brightest stars found. However, astronomers often found bright stars which, based on this theory, should not exist. The current theory suggests that blue stagglers are formed when stars merge together, forming a new, much brighter star. There are other alternative theories, suggesting that blue staggers might come from another place or that somehow they were formed later. Formation Contrary to simulations in Universe Sandbox, stars can merge and they do so without always creating a supernova. In most cases, when two main sequence stars collide, they produce an explosion known as red nova, preserving most of the common mass. Blue stagglers could also form in binary systems, when the two companions are too close. Tidal forces and friction within their atmospheres, as well as gravitational waves, absorb kinetic energy, making the two stars come closer and in the end merge. Alternatively, two stars forming two different systems can merge by chance. This is common in globular clusters and galactic cores, but very rare in places with low stellar densities like our position in the galaxy. Stars also expand at the end of their lifelines (see Supergiant Stars or Red Giants for details) and can swallow another stellar companion. Appearance A blue staggler looks like should look a star of its mass. For stars, luminosity increases exponential to mass. A star whose mass is twice that of the Sun will generate 25 times more light and will also have a much shorter lifespan. They are referred as blue because brighter stars of the main sequence are always bluer then dimmer stars. A special subcategory is represented by red stagglers, which have a greater mass then their neighboring stars, but are at the end of their lifelines and are turning red. Again, these stars could not exist in theory unless they have merged with another star at some point. Such objects are supposed to form when a star approaches the end of its life and increases in size, swallowing a nearby companion. Possible Planets It is theorized that planets usually don't orbit stars in globular clusters. Gravitational interactions would pull a planet out of orbit. However, stagglers can be found (even if more rare) in other parts of the cosmos. Is there any chance that a planet could still be in orbit? In case of a staggler formed by the merger of two passing stars, most planets are expected to be ejected. Those planets who somehow remained in orbit should be on highly elliptical trajectories. Inner planets have the best chance to still be in orbit. In such a system, terraforming becomes very hard or even impossible. See Elliptical Orbit for details. In case of two co-orbiting stars that merge, planets would not have their orbits too affected. However, the red nova will have a dramatic effect on local climate. After the nova, planets will be exposed to a luminosity much stronger then before. This will push the Habitable Zone towards outer planets. It is possible that such an event can actually increase the number of habitable planets.